I will not try to answer point by point as it is late, maybe tomorrow... 
Nevertheless, I wanted to point out several things.

First, I don't understand completely what you are trying to do. If you 
implement an unbiased method, of course the pictures will be radically 
different. There is no way around that. If the pictures, with the exact 
same parameters, were only more grainy, but eventually converged to the 
exact same image on average, then the method would be biased...

The other problem is indeed that many material models within POV-Ray are 
physically inaccurate, so you'll be unable to even use them and still 
make sense. For example, reflection 1 transmit 1 is not realistic. 
Simply follow the light through the surface. 100% of the light should be 
transmitted, and 100% reflected? It's clearly not conservative. I 
suspect the case of your black reflective object is a bit similar.

And of course you cannot use max_trace_level because it introduces a bias...

All of this is why I was thinking that adding such capability into the 
current POV would not be a trivial matter...

-- 
Vincent

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